The Shopping List lets users enter an item and add it to their list.
There's a bug in the current application:
New items replace existing items instead of being appended.
Whitespace-only values can also be added.
Fix the component so that:
- The list starts empty.
- New items are appended.
- Existing items remain.
- The input clears after adding.
- Blank or whitespace-only items aren't added.